A process of producing unsaturated acids from vapor phase C3˜C4 olefins by using a catalyst is a typical process of catalytic vapor phase oxidation.
Particular examples of such catalytic vapor phase oxidation include a process of producing acrolein and/or acrylic acid by the oxidation of propylene or propane, a process of producing methacrolein and/or methacrylic acid by the oxidation of isobutylene, t-butyl alcohol or methyl-t-butyl ether, a process of producing phthalic anhydride by the oxidation of naphthalene or orthoxylene, and a process of producing maleic anhydride by the partial oxidation of benzene, butylene or butadiene.
Generally, catalytic vapor phase oxidation is carried out by charging one or more kinds of granular catalysts into a reactor tube, supplying feed gas into a reactor through a reaction tube, and contacting the feed gas with the catalyst in the reactor tube. Reaction heat generated during the reaction is removed by heat exchange with a heat transfer medium, whose temperature is maintained at a predetermined temperature. The heat transfer medium for such heat exchange is provided on the outer surface of the reaction tube so as to perform heat transfer. The reaction mixture containing a desired product is collected and recovered through a duct, and then sent to a purification step. Since the catalytic vapor phase oxidation is a highly exothermic reaction, it is very important to control the reaction temperature in a certain range and to reduce the size of the temperature peaks at hot spots generated in reaction zones. It is also important to accomplish heat dispersion at a point to be subjected to heat accumulation due to the structure of the reactor or that of the catalyst layer.
The catalysts that may be used to perform partial oxidation of olefins include composite oxides containing molybdenum and bismuth, molybdenum and vanadium, or mixtures thereof.
Generally, (meth)acrylic acid, a final product, is produced from propylene, propane, isobutylene, t-butyl alcohol or methyl-t-butyl ether (referred to as ‘propylene or the like’, hereinafter) by a two-step process of vapor phase catalytic partial oxidation. More particularly, in the first step, propylene or the like is oxidized by oxygen, inert gas for dilution, steam and a certain amount of a catalyst, so as to produce (meth)acrolein as a main product. Then, in the second step, the (meth)acrolein is oxidized by oxygen, inert gas for dilution, steam and a certain amount of a catalyst, so as to produce (meth)acrylic acid. The catalyst used in the first step is a Mo—Bi-based oxidation catalyst, which oxidizes propylene or the like to produce (meth)acrolein as a main product. Also, some acrolein is continuously oxidized on the same catalyst to partially produce (meth)acrylic acid. The catalyst used in the second step is a Mo—V-based oxidation catalyst, which mainly oxidizes (meth)acrolein in the mixed gas containing the (meth)acrolein produced from the first step to produce (meth)acrylic acid as a main product.
A reactor for performing the aforementioned process is provided either in such a manner that both the two-steps can be performed in one catalytic tube, or in such a manner that the two steps can be performed in different catalytic tubes, respectively. U.S. Pat. No. 4,256,783 discloses such a reactor.
Meanwhile, (meth)acrylic acid producers have made diversified efforts to improve the structure of the above reactor so as to increase the production yield of (meth)acrylic acid obtained from the reactor; to propose the most suitable catalyst to induce oxidation; or to improve operating conditions of the process.
As a part of such prior efforts, the high space velocity or the high concentration of propylene or the like supplied into the reactor is used. In this case, there is a problem in that oxidation occurs rapidly in the reactor, making it difficult to control the resultant reaction temperature. There is another problem in that hot spots are generated in the catalyst layers of the reactor and heat accumulation occurs in the vicinities of the hot spots, so that the production of byproducts, such as carbon monoxide, carbon dioxide and acetic acid increases at high temperature, thereby reducing the yield of (meth)acrylic acid.
Furthermore, when (meth)acrylic acid is produced by using propylene or the like to a high space velocity and high concentration, reaction temperature increases abnormally in the reactor, thereby causing various problems, such as the loss of active ingredients from the catalyst layer, or a reduction in the number of active sites caused by the sintering of metal components, resulting in degradation in the quality of the catalyst layer.
Accordingly, in the production of (meth)acrylic acid, control of the reaction heat in the relevant reactor is the most important to ensure high productivity. Particularly, both the formation of hot spots in the catalyst layers and the heat accumulation in the vicinities of the hot spots should be inhibited, and the reactor should be effectively controlled so that the hot spots do not cause the so-called runaway phenomenon of the reactor (runaway: a state in which the reactor cannot be controlled or the reactor explodes due to a highly exothermic reaction). Therefore, it is very important to inhibit the generation of the hot spots and heat accumulation in the vicinities of the hot spots so as to extend the lifetime of catalysts and to inhibit side reactions, and thus to increase the yield of (meth)acrylic acid. To achieve these objectives, many attempts have been steadily made.
Meanwhile, in order to operate the above processes more effectively, the reaction system should be designed in such a manner that it is suitable for oxidation with excessive heat generation. Particularly, in order to inhibit the deactivation of a catalyst caused by excessive heat generation, it is necessary to establish an efficient heat control system capable of controlling extremely high temperatures at hot spots, heat accumulation in the vicinities of the hot spots, and a runaway phenomenon. To provide an efficient heat control system, many studies have been made to establish a circulation pathway of molten salts by mounting various baffles (e.g., U.S. Pat. No. 3,871,445), to design an oxidation reactor integrated with a cooling heat exchanger (e.g., U.S. Pat. No. 3,147,084), to provide a multi-stage heat control structure using an improved heat exchanger system (e.g., Korean patent application No. 10-2002-40043, and PCT/KR02/02074), and to control the structure of a catalyst layer and the reaction temperature, so as to be suitable for an improved heat exchange system (e.g., Korean patent application No. 10-2004-0069117).